Conventionally, in the electrostatic recording process of copying machine, printer, etc., the following print method is employed. First, by uniformly charging the surface of a photo conductor (latent image carrier), then projecting an image from an optical system, and removing charge from a part where is projected the light, an electrostatic latent image is formed. Then, by supplying toner with this electrostatic latent image to form a toner image by electrostatic attraction, and transferring this image to a recording medium such as paper, OHP, developing paper, the printing is completed.
In this case, printing is basically carried out in accordance with the process in color printers and color copying machines, but in the case of color printing, color tone is remanufactured by using four color toners including magenta, yellow, cyan and black. Therefore a process to obtain necessary color tone by superposing these toners in predetermined ratio is required, and several systems for carrying out this process have been presented.
First, there is image on image development in which developing is carried out by superposing the four color toners including magenta, yellow, cyan and black in order similarly to black and white printing, in the process of visualizing an electrostatic latent image by supplying toner on a photo conductor to form a color toner image on the photo conductor. This system enables the comparatively compact apparatus structure. However, controlling gradation is very difficult in this system, so there is a problem that high image quality cannot be obtained.
Second, there is a tandem system that four photo conductive drums are provided, and by developing latent images of each drums with magenta, yellow, cyan and black toner respectively, four toner images including a toner image formed by magenta, toner image formed by yellow, toner image formed by cyan and toner image formed by black are formed, then photo conductive drums formed these toner images are arranged in tandem, each toner image is transfer to a recording medium such as paper in order, and reproduce color image by superposing the images on the recording medium. This system provides a good image, but four photo conductive drums, charge mechanisms and develop mechanisms for each photo conductive drum are arranged in tandem, which increasing the size as well as price of the apparatus.
FIG. 2 shows a structural example of a printing unit in an image forming apparatus of the tandem system. Four printing units, each unit comprising a photo conductive drum 1, an electrostatic roll 2, a developing roll 3, a developing blade 4, a toner supply roll 5 and a cleaning blade 6, are arranged in tandem to correspond with respective toner of yellow Y, magenta M, cyan C and black B, is driven by driving roller (driving member) 9 circularly to transfer the toners in order onto the paper transported by a transfer transport belt 10, and then color image is formed. The transfer transport belt is charged/discharged by a charging roll 7/discharging roll 8, respectively. To charge a paper for attracting the paper to the belt, an attaching roller (not shown) is used. These processes can reduce the occurrence of ozone. The attaching roller feeds the paper from a transport path onto the transfer transport belt, while electrostatically attracting the transfer transport belt. In addition, after transfer, the paper can be separated simply by self stripping that is caused by decreasing transfer electric pressure to decrease the attraction force between the paper and the transfer transport belt.
There are resistive material and dielectric material as the material of the transfer transport belt 10, which respectively has advantages and disadvantages. A belt made of resistive material can hold charge for a short time, so that when being used for transfer in the tandem system, charge injection in transfer is less and electric pressure doesn't go up so much even in the case which four colors are successively transferred. Additionally, charge is discharged when the belt is used repeatedly for transfer to the next paper; therefore electrical reset is not needed. However, resistance value changes depending on environmental variation, which affects transfer efficiency. The belt tends to be affected by the thickness and width of the paper, which is also disadvantage.
Meanwhile in the case of the dielectric material belt, injected charge is not naturally discharged, so that both injection and discharge of charge have to be electrically controlled. However, charge is stably held, hence the attraction of a paper is ensured and paper transport with high accuracy is implemented. Since the dielectric constant has a reduced dependency on the temperature and relative humidity, the transfer process is comparatively stable with respect to environment. Repeating transfer accumulates charge on the belt, therefore transfer electric pressure goes high, disadvantageously.
Third, there is a transfer drum system that a recording medium such as paper is rolled around a transfer drum and rotated for four times, in each rotation magenta, yellow, cyan, and black toner on a photo conductor is transferred in order onto the recording medium to reproduce a color image. This system provides relatively high image quality. However, there is a problem that the type of the recording medium recording medium is limited, since heavy paper such as post card is hard to be rolled on the transfer drum.
As an alternative system for the image on image print system, the tandem system and the transfer drum system, the intermediate transfer system that is capable of obtaining high quality image while not increasing the size of apparatus nor limiting the type of recording medium is presented.
In other word, in this intermediate transfer system, an intermediate transfer member including a drum or a belt which temporally transfers and holds a toner image on a photo conductor is provided, and around the intermediate transfer member, four photo conductors, respectively forming a toner image formed by magenta, a toner image formed by yellow, a toner image formed by cyan and a toner image formed by black, are provided, by transferring the four-color toner images onto to the intermediate transfer member in order, a color image is formed on the intermediate transfer member to transfer the color image onto a recording medium such as paper. Since the system adjusts gradation by superposing the four-color toner images, it is possible to obtain high image quality, at the same time, no needs to arrange photo conductors in tandem like the tandem system doesn't specially increase the size of apparatus, and moreover, no need to roll a recording medium around the drum doesn't limit the type of recording medium.
As an example apparatus forming color images by intermediate transfer system, an image forming apparatus using an endless belt like an intermediate transfer member as an intermediate transfer member is illustrated in FIG. 3.
In FIG. 3, numeral 11 is a drum type photo conductor and is configured to rotate in the arrow direction in the figure. This photo conductor 11 is charged by a first charge unit 12, and then charge on a portion exposed by image exposure 13 is eliminated, then an electrostatic latent image corresponding to a first color component is formed on the photo conductor 11, further the electrostatic latent image is developed by a developing unit 41 with the first color magenta toner M, and the first color magenta toner image is formed on the photo conductor 11. Next, this toner image is driven by a driving roller (driving member) 30 cyclically to be transferred to cyclically rotating an intermediate transfer member 20 while being in contact with the photo conductor 11. In this case, the transfer from the photo conductor 11 to the intermediate transfer member 20 is carried about by a first transfer bias that is applied from a power supply 61 to the intermediate transfer member 20 at a nip portion between the photo conductor 11 and the intermediate transfer member 20. After the first color magenta toner image is transfer to the intermediate transfer member 20, the photo conductor 11 is cleaned its surface by a cleaning unit 14, and the photo conductor 11 completes the first rotation of the develop and transfer operation. After then, the photo conductor rotates for three times, a second color cyan toner image, a third color yellow toner image and a fourth color black toner image are formed in order on the photo conductor 11 by use of developing units 42-44 respectively, and these toner images are superposing transferred to the intermediate transfer member 20, then a combined color toner image corresponding to a target color image is formed on the intermediate transfer member 20. For the apparatus in FIG. 3, each developing units 41-44 is configured to develop with magenta toner M, cyan toner C, yellow toner Y, and black toner B in order and in turn for every rotation of the photo conductor 11.
Next, a transfer roller 25 comes to contact with the intermediate transfer member 20 formed the combined color toner image, and a recording medium such as paper 26 is fed from a paper feeding cassette 19 to a nip portion between the roller 25 and the member 20. Simultaneously, a second transfer bias is applied from a power supply 29 to the transfer roller 25, the combined color toner image is transferred from the intermediate transfer member 20 to the recording medium 26 to make a final image by heat fusing. The intermediate transfer member 20 after transferring the combined color toner image to the recording medium 26 is removed transfer resident toner on surface thereof by a cleaning unit 35 and is configured to recover to the initial condition for preparing the next image forming.
Also, there is a tandem intermediate transfer system which comminutes the tandem system and the intermediate transfer system. FIG. 4 illustrates an image forming apparatus of the tandem intermediate transfer system for forming color images by use of an endless belt type tandem intermediate transfer member.
In the illustrated apparatus, a first developing unit 54a to a fourth developing unit 54d for developing respective electrostatic latent image on photo conductive drums 52a to 52d with yellow, magenta, cyan, and black are arranged along with a tandem intermediate transfer member 50, and by cyclically driving this tandem intermediate transfer member 50 in the arrow direction in the drawing to successively transfer four-color toner images formed on the photo conductive drums 52a to 52d of respective developing unit 54a to 54d, thereby a color toner image is formed on the tandem intermediate transfer member 50, and by transferring this toner image onto the recording medium such as paper 53, printout is completed. In this case, in either apparatus described above, the arrangement order of the toners used for developing is not limited and can be optionally selected.
Numeral 55 in the drawing indicates a driving roller or a tension roller for cyclically driving the tandem intermediate transfer member 50, numeral 56 indicates a recording medium feeding roller, numeral 57 indicates a recording medium feeding unit, and numeral 58 indicates a fuser unit for fusing an image onto a recording medium by heat and so on. Additionally, numeral 59 indicates a power supply unit (electric pressure applying means) for applying electric pressure to the tandem intermediate transfer member 50. This power supply unit 59 is configured to be capable of reversing positive and negative of applying electric pressure depending on the case where a toner image is transferred to the tandem intermediate transfer member 50 from photo conductive drums 52a to 52d or the case where a toner image is transferred to a recording media 53 from the tandem intermediate transfer member 50.
Conventionally, for conductive endless belts used such as the endless belt type transfer transport belt 10, the intermediate transfer member 20, and the tandem intermediate transfer member 50, etc., semi conductive resin film belt and rubber belt having fiber reinforced member are mainly used. From these, as an example of the resin film belt, in Japanese Published Examined Patent Application No. H8-7505, a belt blended thermoplastic resin and polyether ester amide or polyether amide and sulfonate metal salt or sheet member is disclosed. Also, Japanese Published Unexamined Patent Application No. 2004-272209 discloses a belt that is formed by copolymers having polyether block in elastomer polyester, positive ion and negative ion adding disaggregatable salt, and melamine cyanurate as fire retardant, and Japanese Published Unexamined Patent Application No. 2000-62993 disclosed a belt including conductive base layer which is formed by dispersing conductive filler to polyester polyether resin with fusing point within a range from 160 to 210° C. and has predetermined loss tangent (tan δ) and volume resistive value.
Furthermore, Japanese Published Unexamined Patent Application No. 2003-29537 discloses a belt that is formed by thermoplastic polybutylene terephthalate (PBT) resin and/or thermoplastic PBT elastomer adding polymer ion conductive agent, and Japanese Published Unexamined Patent Application No. 2005-62822 discloses a belt that is formed by PBT and polyester-based thermoplastic elastomer adding thickener and carbon black.
Also, Japanese Published Unexamined Patent Application No. 2002-132053 discloses a conductive endless belt using thermoplastic polyalkylene naphthalate resin or polymer alloy combined the resin and another thermoplastic resin, or polymer blend as a base.
In above mentioned Japanese Published Examined Patent Application No. H8-7505 and Japanese Published Unexamined Patent Application No. 2004-272209, belts added polymer ion conductive agent are disclosed, and it is described that by further adding metal salts, environment dependency that is a disadvantage of polymer ion conductive agent is improved. However, in this case, low melting thermoplastic elastomer is used for the base resin, accordingly elastic modulus of the belt is low and creep resistance has no advantage, so that there is a problem that belt peripheral length is changed by repeat use, which causes problem in the image.
Furthermore, in the art disclosed in Japanese Published Unexamined Patent Application No. 2000-62993, elastomer polyester with low fusing point is used, therefore a similar problem in above mentioned Japanese Published Examined Patent Application No. H8-7505 and Japanese Published Unexamined Patent Application No. 2004-272209 occurs, as well as a problem that resistance variation in belt circumferential direction is high since carbon black is used as a conductive agent.
Moreover, in Japanese Published Unexamined Patent Application No. 2003-29537, PBT and/or polyester-based elastomer which has high elasticity is used, but compatibility among polyether esteramide, PBT and elastomer polyester in polymer ion conductive agent is too good, so that unless added quantity of polyether esteramide is increased, predetermined conductive isn't seen, and as a result, the belt has low elastic modulus. In addition, in Japanese Published Unexamined Patent Application No. 2005-62822, thickener was added to improve dispersibility of high viscosity PBT and low viscosity elastomer polyester, which decreased residence variation, however, since carbon black was used as a conductive agent, the result was not necessarily satisfied.
As described above, any art priory suggested could not fully satisfy the desired performance for the belt, hence implementing higher quality belt with desired elastic modulus and creep resistance as well as with less volume resistance variation and environment dependency has been desired.
On the other hand, in the case which the belt rotating position in driving is controlled by an optic sensor, glossiness of the belt surface is important. In this respect, when a belt is manufactured in a molding method using a mold, the surface nature of the belt can be controlled by the mold, therefore there are less problem, but when a belt is manufactured only in extruding molding, (ref: Japanese Published Unexamined Patent Application No. 2001-138380), effect to the surface nature due to belt material increases. On the other hand, the belt disclosed in Japanese Published Unexamined Patent Application No. 2002-132053 has high glossiness, but has a problem in belt durability due to insufficient flexibility, therefore the crystallization of resin is progressed as using under high temperature and high humidity condition, which deteriorates the setting performance.
Additionally, since the belt disclosed in Japanese Published Unexamined Patent Application No. 2000-62993 uses elastomer with low fusing point, the durability thereof is good, but the belt has problems such as offset image due to occurrence of elongation and image degrading due to deformation under high temperature and high humidity condition.